Homogeneous or non-supported metallocene catalysts, cocatalyzed by organoaluminum compounds (usually methylaluminoxane), are well known. They may be exploited for their high catalytic activity when used in olefin polymerization and for their ability to produce polymers with terminal unsaturation. However, these homogeneous catalyst systems have several disadvantages. They require very high cocatalyst (methylaluminoxane) to catalyst (metallocene) ratios and the polymers made with these catalysts are known to foul their reactors. Further, these catalysts do not offer desirable resin morphology and produce relatively large amounts of undesirable fines. All these disadvantages limit the commercial utility of homogeneous metallocene catalyst systems.
On the other hand, supported catalyst systems improve resin morphology and are effective with lower cocatalyst to catalyst ratios. However, the known supported systems are typified by low catalystic activity. A side effect of this low catalytic activity is unacceptable catalyst residue in resin produced with such systems. These residues cause processing problems and deteriorate the quality of the end products. The low catalytic activity of these supported catalysts systems has caused start-up problems for fluid-bed processes, and has resulted in unfavorable process economics. Moreover, the metallocene tends to leach off the support of previously known systems. Therefore, the synthesis of a supported metallocene/aluminoxane catalyst system that offers commercially acceptable levels of catalyst activity, desirable resin morphology, and which does not cause reactor fouling during polymerization has remained a challenge.
Prior attempts to make supported metallocene catalyst systems with high activity and which produce polyolefins having uniform particulate morphology and narrow particle size distribution, include the following: preactivate or precontact the metallocene and the organoaluminum cocatalyst (EP 302,424; EP 354,893); prepolymerize a heterogeneous catalyst system using at least one olefin (EP 426,646; U.S. Pat. No. 4,871,705); modify a dehydrated hydroxyl-containing inorganic support before loading the metallocene catalyst (WO 200226842); and conducting the polymerization using an aromatic solvent (U.S. Pat. No. 6,468,936). Another approach which has been used is to modify an inorganic support in such systems using: the methyl aluminoxane (MAO) cocatalyst (U.S. Pat. No. 4,808,561); organosilicon-containing compounds (U.S. Pat. Nos. 4,874,734; 5,206,199; 5,627,246; 6,194,343; 6,239,059; alumino-siloxane compounds (U.S. Pat. No. 5,206,199), and a mixture of borax and methanol (U.S. Pat. No. 6,239,060).
The prior attempts to make an improved supported metallocene catalyst have not been entirely successful in that they have resulted in polymer products having irregular and broad particle size distribution and poor resin morphology. Others have produced better resin morphology; but have achieved lower than desirable catalyst activity. Although the aromatic hydrocarbon solvents such as benzene, toluene, and substituted benzene were found to increase the activity of supported metallocene catalyst systems, the use of these solvents has caused metallocene leaching and dissolution. Accordingly, the prior art approaches to supported metallocene catalysts systems have suffered from one or more of the following deficiencies: low catalyst activity, unacceptable levels of reactor fouling during polymerization, the production of polymers having fines and/or broad molecular weight distributions.
The present invention applies a novel approach to decrease the steric hindrance and prevent bimetallic deactivation in supported metallocene catalyst systems through the use of organogermane and/or organotin support modifiers. It also solves the problems of low catalyst activity and unacceptable levels of reactor fouling during polymerization. Further, it achieves these results using inexpensive and simple-structured metallocenes and only small amounts of aluminoxane cocatalyst. The catalyst system of the present invention demonstrates good hydrogen responsiveness, particularly using a very small amount of hydrogen. The high activity of the present catalyst system enables efficient fluid-bed polymerization and the low cocatalyst to catalyst ratios which are possible with the invention result in polymers having minimal contamination.